We shall continue our studies of cyclophosphamide derivatives, we shall investigate the molecular structures of some folic acid antagonists which are potentially useful anti-cancer agents, and we shall refine the structure of the thymidylyl-(5',3')-thymidylate-(5') dinucleotide (pTpT). Specific tasks we shall undertake are: (1) Crystal structure analysis of 4-hydroperoxytrophosphamide. (2) Structure determinations of some of B. R. Baker's folic acid antagonists. We presently have usable crystals of two such compounds and will undertake data collection in the near future. (3) Refinement of the structure of pTpT. This compound is the only dinucleotide whose molecular structure has been elucidated. Refinement of the structure will yield much useful data on conformational parameters for nucleic acids.